I just got done watching a fascinating youtube video asking Stanford students to solve the problem 18 x 5 without pen and paper.
The results? Let's just say the answers may have all been the same, but the methods varied widely.
One student was able to halve the number 18 to make it a 9 instead. Then he multiplied 9 X 5. Then multiplied 9 X 5 again, finally adding the two together.